The present invention relates to a contact plug of a semiconductor device and, more particularly, to a method of forming the contact plug, which can prevent defects from occurring when contact holes are gap-filled with a conductive material.
Flash memory semiconductor devices are one type of nonvolatile memory which can retain its data when power is turned off. Flash memory can be electrically programmed and erased and does not need the refresh function for rewriting data at periodic intervals. This flash memory device is largely classified into NOR flash memory and NAND flash memory depending on the structure and operating conditions of a cell. NOR flash memory has a plurality of word lines connected in parallel and enables program and erase on a specific address. Thus, the NOR flash memory is generally used for application fields requiring high-speed operation. On the contrary, NAND flash memory has a structure in which a plurality of memory cell transistors are connected in series to thereby form one string and is generally used for application fields needing highly-integrated data retention.
In a NAND flash memory, a semiconductor substrate having the source/drain regions formed therein and source/drain contact plugs connecting metal lines are generally formed by forming an insulating layer between the semiconductor substrate and the metal lines, forming contact holes in the insulating layer, and gap-filling the contact holes with a conductive material to thereby form the source/drain contact plugs. However, as memory becomes highly integrated and ultra-miniaturized, the width of the contact hole is gradually narrowed. Thus, bowing can be generated when forming the contact hole, or an overhang profile can occur while the contact holes are gap-filled with the conductive material, so that a seam is generated within the contact plugs. In particular, this overhang profile can be generated when depositing a barrier metal layer on the contact holes. An upper portion of the contact hole is narrowed due to the overhang profile of the barrier metal layer. Consequently, when the barrier metal layer is formed within the contact holes in a subsequent process, a seam is generated within the contact plugs. The seam generated as described above becomes even larger in a subsequent etch or polishing process, which may result in reliability problems.